American Fisheries Society. 149 
of the lake it shows 11.8°. The oxygen shows an arrangement 
corresponding to this division of the lake into two parts; a warm 
lake 7 to 8 meters in thickness, kept in circulation by the wind 
and floating on top of the lower water of the lake, whose greatest 
depth is about 16 meters and which is cut off from the air by the 
upper stratum. There are about 6 cc. of oxygen per liter in the 
upper water, but that in the lower water shows the effect of the 
cutting off of the lower water from new supplies and is nearly 
exhausted. Its exhaustion is in small part due to the use of oxy- 
0 50 iO 15 MOO T2425 








. cf ; 
FiG. 6—Lake Mendota, Aug. 31, 19U5. 
gen for respiration by the animals living in deep water. In far 
greater part it is used up by decomposition of plants and animals. 
Down into this lower water are sinking all of the minute dead 
plants and animals, as well as the larger ones, from the surface 
of the water; into it is also sinking the debris from along the 
shore. All of this is decomposed in the bottom water, inevitably 
using up the supply of oxygen; and that process is showing its 
effect very plainly from the 7 meter line down. You will see at 
the 7 meter level the oxygen line begins to curve towards zero 
and at 13 meters there is very little oxygen left in the water, 
